[texhax] `\vtop to 0pt' produces a vertical shift: is that normal?

[Rodolfo Medina]

With plain TeX: if I put a `\vtop to 0pt{}' after a `\par' command, that
produces a vertical shift, whereas one would expect a vertical box of height
zero to not produce any visible vertical effect at all.

Why does this happen, and how to work it out?

Thanks for any help
Rodolfo